The invention relates to a sensor for the capacitive measurement of film thicknesses, with a drum, which rolls along the film and has at least one measuring capacitor, the plates of which are disposed in the peripheral surface of the drum, so that its capacitance is effected by the thickness of the film, and with a transformer, for transmitting the measurement signal from the rotating drum to a stationary part.
Such sensors are used particularly for measuring the thickness of blown films. The drum is then disposed in such a manner at the periphery of the film bubble, which is extruded from the extrusion die and inflated with inflating air, that it rolls along the film bubble, which is pulled off in the upward direction. The advantage over a stationary sensor consists therein that there is no friction between the measuring head and the film and therefore scratching of the film is avoided. If the sensor furthermore can be rotated about the vertical axis of the film bubble, the thickness profile of the whole periphery of the film bubble can be recorded during a complete revolution of the sensor. The thickness profile, so measured, can then be used, for example, to regulate the thickness of the film with the help of a cooling ring in a closed regulating circuit, which can be controlled segmentally and is described, for example, in EP-A-O478 641.
However, the spatial resolution of the thickness measurement is limited owing to the fact that only one measurement per revolution of the drum can be recorded with each measuring capacitor, since the measurement can be made only with the measuring capacitor in contact with the film. In order to achieve a high resolution, the periphery of the drum should therefore be as small as possible. On the other hand, however, the least size of the periphery of the drum is limited owing to the fact that the measuring capacitor must have a certain minimum size, so that an adequately accurate measurement signal is obtained.
U.S. Pat. No. 5,223,797 discloses a sensor of the type named above, the drum of which has two diametrically opposite measuring capacitors. The capacitor plates and the capacitor gap, formed between them are oriented at right angles to the axis of the drum. The opposite ends of the drum are mounted rotatably in a fork and the transformer for decoupling the measurement signal is formed by two sliding contact units, which are disposed at opposite ends of the drum and centered on the axis of the drum. One of the sliding contact units is connected with the respective positive plates of the measuring capacitors and the other sliding contact unit is connected with the negative or ground plates of the measuring capacitors. If a measuring capacitor moves past a film, the electrical edge field, formed between the capacitor plates, penetrates into the film material, so that the film acts as a dielectric and therefore the capacitance of the measuring capacitor is affected by the thickness of the film. With the help of the transformer, formed by the sliding contact units, the voltage, which depends on the capacitance of the measuring capacitor, is transferred to the fork and, from there, further to a stationary evaluating circuit. The fork, in turn, can be mounted rotatably, so that the drum can roll on a helical path relative to the film, while the measuring device revolves around the film bubble.
However, there is a problem with the known measuring device in that the effective capacitance of the measuring capacitors, and, with that, also the voltage, serving as a measurement signal, can easily be distorted by external influences, such as by stray electrical fields or by changes in the geometry of electrically conducting parts in the surroundings of the conducting connections between the measuring capacitors and the evaluating circuit. These interfering effects can be suppressed only inadequately by shielding measures.
Basically, in the case of capacitive measuring systems, it is known that an oscillating circuit can be built up with the help of the measuring capacitor and the intrinsic frequency of the oscillating circuit, which depends on the capacitance of the measuring capacitor, can be evaluated as a measurement signal. In this case, however, the problem exists that the intrinsic frequency of the oscillating circuit can also be distorted easily by external influences. Changes in the geometry of the electrical leads, forming the oscillating circuit, can also in this case easily lead to a distortion of the measurement signal.
DE-A-33 35 766 discloses a measuring device, with which the thickness of an electrically conductive coating of a tape can be measured. The tape is diverted at a drum, in the peripheral surface of which several capacitor plates are disposed offset in the peripheral direction and distributed over the length of the drum. The electrically conductive layer is on the side of the tape, averted from the surface of the drum, so that, with one of the capacitor plates, it forms a capacitor, the dielectric of which is formed by the tape itself With the help of a high frequency generator, alternating voltage is applied to the capacitor plates and the measurement signal is supplied to a transformer, which is accommodated in the drum, and transferred to there, for example, by wireless means, to a stationery part. With the help an angle transmitter, the respective angular position of the drum is determined, so that the measurement signal obtained by the transducer can be assigned to the individual capacitor plates and the profile of the layer thickness can thus be determined over the whole width of the tape.
However, the thickness of the layer does not affect the capacitance of the measuring capacitor. Rather, the principle of measurement is based upon the fact that the detuning of the measurement signal is determined by the electrical connectivity of the layer, which in turn, depends on the thickness of the layer. Accordingly, since the conductivity and not the capacitance is measured, the measuring device is relatively insensitive to external effects. However, this principle of measuring cannot be employed for measuring the thickness of non-conductive films.
It is an object of the invention to provide a sensor of the type named above, which makes possible a more accurate measurement of film thickness, and one which largely is not distorted by external effects, with a high special resolution.
Pursuant to the invention, this objective is accomplished by an oscillator, which is integrated in the drum and produces a frequency signal, which depends on the capacitance of the measuring capacitor.
Accordingly, pursuant to the invention, the oscillator and measuring capacitor form an oscillating circuit, the oscillating frequency of which depends on the capacitance of the measuring capacitor and, with that, on the thickness of the film in contact with this measuring capacitor. Since the oscillator is integrated in the drum, the geometry of the electrical leads forming the oscillating circuit is not changed by the rotation of the drum, so that the measurement signal is not distorted by the geometry of the leads. In addition, the leads forming the oscillating circuit, can be kept very short, so that the sensitivity of the oscillating circuit towards external noise signals and external damping is also decreased drastically. Since the measurement signal is a frequency signal, it is not affected either by static fields, which result, for example, from static charges. The measurement signal can be passed on directly as a frequency signal, optionally after a frequency conversion with the help of the transducer. Even if the transducer were to be exposed to external interfering effects, this would usually lead to hardly any distortion in the frequency of the measurement signal, so that finally, in the stationary part of the measurement arrangement, a largely undistorted signal is obtained. Alternatively, the measurement signal, before it is passed on to the transformer, can also be converted into a different signal, such as a voltage or current signal.
In this way, it is possible to carry out a sufficient sensitive and accurate thickness measurement with a measuring capacitor, which has relatively small dimensions. Accordingly, the drum can also have a relatively small periphery, so that a high spatial resolution of the measurement is achieved. The electronic components, required for producing the capacitance-dependent frequency signal, can be constructed small, for example, as integrated circuits, so that they can be accommodated in the drum without problems in spite of the small dimensions of the drum.
In a preferred embodiment, the drum contains several, for example four measuring capacitors, so that the spatial resolution correspondingly is quadrupled. Such a drum can be produced easily and rationally owing to the fact that the measuring capacitors are formed by strip conductors on a common multilayer plate, which is oriented at right angles to the axis of the drum. Electronic components integrated in the drum, can also then be disposed on this plate.
For this construction, the capacitor plates and the gaps formed between them extend in the peripheral direction of the drum. Their length in the peripheral direction of the drum is, however, preferable smaller than the length of arc, on which the film lies against the periphery of the drum. By these means, it is ensured that for each revolution of the drum, there is a sufficiently long period of time, during which the film lies in contact with the periphery of the drum over the whole length of the capacitor plates, so that distortion of the measurement signals due to incomplete contact between film and drum is avoided.
Preferably, the measuring device has an angle measuring device for measuring the angular portion of the drum and the individual measuring capacitors are controlled by means of the measured angular position so that, in each case, only that particular measuring capacitor is active, which happens to be in contact with the film. Mutual interference by the oscillating circuits is avoided so that at all times not more than one oscillating measurement circuit is active. An interference-free measurement is thus possible even when a relatively large number of measuring capacitors are accommodated on a relatively small periphery of the drum.
The angle-measuring device also offers the possibility of activating each measuring capacitor additionally during the short period, in which it is not in contact with the film and in which also none of the other measuring capacitors is active. The measurement signal obtained in this time can be used to calibrate the measuring capacitor in question. Since each measuring capacitor can be calibrated anew during each revolution of the drum, measurement errors, which arise out of the drift in the static capacitance of the measuring capacitor or out of mechanical or electrical changes in the sensor, can be eliminated continuously. The difference between the resting frequency of the measuring capacitor, measured in the calibration phase and the peak value of the frequency obtained during the actual measuring time, can then be evaluated as the measurement value. The temperature compensation, which continues to be necessary, is also simplified appreciably in this manner. It is sufficient to measure the temperature of each individual measuring capacitor or the temperature of the multilayer plate as a whole and to multiply the measured frequency difference by a correction factor, which represents the temperature-dependent change in the sensitivity, which is brought about by the changing distance between the plates. In the temperature range in question, this correction factor is almost a linear function of the temperature.
Preferably, the frequency signal, generated by the oscillator, is not transferred directly to the transformer; instead, it is first mixed with the frequency signal of a reference oscillator. The signal, finally passed on by the transformer, is then a low-frequency beat signal, which is not susceptible to interference and can be evaluated easily and the frequency of which corresponds to the difference between the frequency of the reference oscillator and the intrinsic frequency of the oscillating circuit, formed by the oscillator and the measuring capacitor.
Preferably, every measuring capacitor has its own oscillator assigned to it, the frequency signal of which is superimposed on the mixer only during the measuring phase.
In a particularly preferred embodiment, the capacitance-dependent frequency signal of the oscillator is coupled into the lead, which also supplies the oscillator with the operating DC voltage. Accordingly, the oscillator can be disconnected and, at the same time, the connection between the oscillator and the mixer cut by opening a single switch.